Not Applicable
Not Applicable
Not Applicable
1. Field of the Invention
The invention generally relates to measuring and testing and specifically to instrument proving and calibrating. In greater detail, the invention is applied to set point adjustment, e.g., zero correction. In a further aspect, the invention is applied to measuring and testing; to instrument proving and calibrating; to volume of flow, speed of flow, volume rate of flow, or mass rate of flow; with signal processing, set point adjustment, e.g., zero correction. The invention is a method and apparatus for the dynamic calibration of a pressure transducer passing a signal whose output strength is proportional to applied pressure or to pressure differential.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The flow rate of a fluid can be detected and monitored by measuring a parameter that varies in a corresponding way to the flow. Wind speed, for example, commonly is monitored by a cup anemometer, which rotates in a wind stream in proportion to wind velocity. The mechanical movement of the cups can be translated into a signal indicative of wind speed by known systems that employ conventional electrical, magnetic, and mechanical means. Because a cup anemometer relies upon moving parts, its accuracy and reliability are degraded over time by mechanical wear and corrosion. Adverse climates can damage a mechanical moving system quite rapidly, such as by sand abrasion in desert areas or icing in high mountainous regions. In addition, a moving mechanical system tends to have limited accuracy, especially at low fluid flow rates where mechanical friction becomes an increasingly large source of error.
In order to overcome the problems inherent in a mechanical system, various types of motionless anemometers have been developed. Some of these operate on the principal of thermal diffusion, by monitoring a heated wire in a wind stream. Such a wire has a known temperature coefficient of resistance. The rate of heat loss, which may correspond to the electrical energy needed in order to sustain a specified resistance, provides a measurable parameter corresponding to wind speed. However, thermal anemometers also face environmental deterioration due to deposits of dirt and other atmospheric pollutants. Over time, they are known to become less accurate in detecting wind speed, particularly at low wind velocity. Another variety of anemometer employs one or more pressure transducers for determining differential pressure between a static pressure and a measured pressure, which result can be correlated with wind speed. The directly measured parameter in the pressure cell is capacitance. The sensitivity of a pressure transducer also tends to decrease over time, typically due to aging, accumulations of dirt and deposit of other atmospheric contaminants. There are still other known systems for measuring wind velocity, although substantially all employ electronic components in order to obtain a sensor reading and interpretation. Loss of accuracy over time remains a persistent problem, particularly at low velocity readings. Both deposits on the sensors and aging of all circuit components contribute to loss of sensitivity and read-out error.
Transducers are calibrated at the time of manufacture so that one or more correction values that will be applied to the output of the transducer. These correction values deal with factors such as ambient temperature, which can cause zero point drift. Overall, the manufacturer attempts to fingerprint the transducer""s performance over its operating range and then compensate for any discovered inaccuracy. Fingerprinting is conducted under controlled temperature conditions. A modem transducer is associated with a processor that is programmed to apply corresponding correction factors to the output signal. Although fingerprinting can add accuracy to the transducer""s output performance based upon the transducer""s characteristics at the time of manufacture, fingerprinting cannot correct for aging and for perhaps other factors. Thus, although it is known to correct the zero level by compensating for output levels as a function of temperature, in practice the data supporting the correction is created in a static, one-time event, such as at the time of manufacture. It would be possible to recalibrate a transducer during its life, such as by returning it to the manufacturer for adjustment. However, this is impractical because the transducer has been installed in an operating device deployed to the field. Further, it may not be evident that a transducer in field use is in need of recalibration. Various methods and circuitry used in field equipment attempt to preserve the accuracy of a transducer by taking into account common sources of inaccuracy. For example, differential pressure signals have been corrected in the field to compensate for inaccuracy in reading ambient temperature and static pressure.
In real time usage, a digital computer may receive the signal from one or more pressure sensors and process the signals to produce an improved resulting signal. U.S. Pat. No. 4,598,381 to Cucci uses a digital computer, serving as a correction circuit, to compare a reference pressure signal to a differential sensor signal. The computer adjusts the reference signal and provides an improved output signal as a function of the differential sensor signal and the adjusted reference signal.
Another correction method is shown in U.S. Pat. No. 5,623,101 to Freitag, which corrects for inaccuracy caused by xe2x80x9cdisturbance variables,xe2x80x9d namely temperature and static pressure. Those variables influence absolute pressure and thereby reduce the accuracy of a corrected differential pressure signal. Freitag recursively calculates a corrected differential pressure signal from a measured differential pressure signal, a measured absolute-pressure signal and a temperature signal in combination with a plurality of lower degree correction polynomials. The corrected differential pressure signal may be further processed via a linearization polynomial to produce a linearization correction signal which, when combined with the corrected differential pressure signal, produces a linearization differential pressure signal.
Another such method is shown in U.S. Pat. No. 5,383,345 to Berard et al., which addresses inaccuracy caused by differences in temperature across a cell used to sense pressure. According to the method, a microprocessor calculates a dynamic temperature factor signal by multiplying a signal representative of temperature change across the cell by a coefficient based on measurements made of the effect of temperature change on the cell. The dynamic temperature factor signal is then subtracted from the signal representative of the differential pressure sensed by the cell to thereby provide the dynamic offset compensation.
U.S. Pat. No. 5,329,818 to Frick corrects the output signal of a differential pressure sensor for errors in static pressure due to changes in temperature.
It would be desirable to create an apparatus and method able to directly correct for aging and other environmental deterioration of a transducer. In particular, it would be desirable to provide a compensation algorithm capable of setting the zero level at a predetermined or ideal signal level. Further, it would be desirable to determine a contemporaneous correction factor as a function of temperature and then apply such factor to the output signal during active measurements of applied pressure.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method and apparatus of this invention may comprise the following.
Against the described background, it is therefore a general object of the invention to correct the output of a differential pressure transducer by employing a compensation routine that can determine the xe2x80x9czeroxe2x80x9d signal during times when a calm condition exists and can store these values as a function of temperature for use during windy and varying temperature conditions.
According to the invention, a processor in operative communication with a transducer applies a dynamic compensation algorithm. The processor continuously re-determines the necessary correction factor to set the zero level of the output signal at a predetermined or ideal zero signal point.
According to another aspect, the correction factor is determined as a function of temperature and stored in non-volatile memory. The correction factor corresponding to ambient temperature is applied to the output signal when the transducer is detecting an applied differential pressure. The correction factor is applied across the entire range of detected differential pressures.
According to a further aspect of the invention, an algorithm determines when correction factors should and should not be calculated or retained for future use. The algorithm takes the time derivative of the differential pressure. At large values of the time derivative, zeroing calculations are disengaged, and the algorithm obtains correction factors from those previously determined and stored in non-volatile memory. At small or zero values of the time derivative, the zeroing calculations are resumed.
The invention provides a method of dynamically compensating a process signal over time for a bias. First, the method monitors the first time derivative of the process signal over time. Next, the method determines from the first time derivative a first time period of predetermined length when the process signal is at zero signal level. Third, the method derives a correction factor for signal bias from the process signal during the first time period. Fourth, and subsequent to the first time period, the method determines a second time period when the process signal is not at zero signal level. Fifth, the method compensates the process signal for bias by applying the signal correction factor to the process signal over the second time period.
According to a further aspect, the process signal is monitored over time to additionally determine a time period of predetermined length during which the process signal is continuously below a predetermined threshold value.
The offset in the process signal is caused by a disturbance signal that is variable in response to a change in value of a disturbance variable. Therefore, the monitoring of the process signal over time is performed by monitoring the process signal as a function of the value of the disturbance variable over time. After deriving a signal correction factor, the method calls for recording the signal correction factor as a function of the value of the disturbance variable during the first time period. When a signal correction factor is to be applied to the process signal over the second time period, the signal correction factor selected from recorded values is a function of a recorded value of the disturbance variable that is equal to the monitored value of the disturbance variable during said second time period.
Determining when the process signal is substantially constant in magnitude over a time period can be done by taking a time derivative of the process signal. Typically, the process signal is a differential pressure signal, the disturbance signal is a zero bias, and the disturbance variable is temperature.
Another aspect of the invention is a method of dynamically calibrating a pressure transducer that produces a pressure output signal indicative of applied differential pressure. The pressure transducer is associated with a processor having access to a memory that is storing correction factors as a function of temperature. The processor applies a correction factor selected from the memory to the output signal of the transducer. The method monitors the pressure output signal over a selected time period by generating a signal representative of a first time derivative of the pressure output signal with respect to the selected time period. Ambient temperature also is sensed during the selected time period. From the time derivative signal, the method can determine whether the time derivative signal is within a preselected threshold value. The method responds to a determination that the time derivative signal is within the preselected threshold value for the selected time period by averaging the pressure output signal over the selected time period; calculating a signal bias of the averaged output signal as a correction factor; and recording the calculated correction factor as a function of temperature in memory.
The method responds to a determination that the time derivative signal is not within the preselected threshold value for the selected time period by recalling from memory a recorded correction factor corresponding to sensed temperature. The recorded correction factor is applied to the pressure output signal over the selected time period to compensate the pressure output signal for bias
The invention is embodied in a dynamically calibrated differential pressure anemometer, in which an anemometer body is associated with suitable hardware, electronics, and software for (1) generating a process signal in response to movement of a fluid with respect to the anemometer body; (2) monitoring the process signal over time to determine a first time period of predetermined length during which the process signal is substantially constant in magnitude; (3) deriving a signal correction factor from the magnitude of the process signal during the first time period; (4) determining a second time period when the process signal is not substantially constant in magnitude over time; and (5) applying the signal correction factor to the process signal over the second time period to produce a compensated process signal.
The anemometer includes hardware, electronics, and software for monitoring the process signal over time to determine a time period of predetermined length during which the process signal is below a predetermined threshold value.
The anemometer body defines a fluid passageway having a constricted or Venturi-like throat producing relatively lower pressure of moving fluid in the constricted throat. A low-pressure tap communicates with the constricted throat, and a high-pressure reference tap is offset from the constricted throat. A pressure transducer is connected in operative communication with the low-pressure tap and high-pressure tap, producing a process signal corresponding to the pressure difference between the taps.
The anemometer employs suitable hardware, electronics, and software to monitor the process signal over time by determining the time derivative of the process signal.
Similarly, the anemometer employs suitable hardware, electronics, and software to derive a signal correction factor by averaging the process signal over the first time period.
In a differential pressure anemometer employing a pressure transducer, the process signal typically is offset due to a disturbance signal that is variable in response to a change in value of a disturbance variable. In order to dynamically calibrate this anemometer, suitable hardware, electronics, and software are provided to monitor the process signal over time as a function of the value of the disturbance variable over time. In addition, the anemometer includes suitable equipment for recording the determined signal correction factors as a function of the value of the disturbance variable during said first time period. Correspondingly, the anemometer includes equipment such as hardware, electronics, and software for applying the signal correction factor to the process signal over the second time period by applying a signal correction factor determined while the recorded value of the disturbance variable is equal to a monitored value of the disturbance variable during the second time period.
A pressure transducer generates the process signal in response to movement of a fluid with respect to the anemometer body. Thus, the transducer generates a signal that is indicative of sensed fluid pressure.
The disturbance signal is a product of the temperature of the transducer and varies with changes in the value of temperature. Accordingly, the anemometer includes a temperature sensor that generates a disturbance variable signal corresponding to sensed temperature values of the pressure transducer.
The anemometer employs a non-volatile memory that receives and records signal correction factors as a function of concurrent temperature values of the pressure transducer.
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with the description, serve to explain the principles of the invention. In the drawings: